Power density and component costs are key performance metrics of both isolated and non-isolated DC-DC power converters to provide the smallest possible physical size and/or lowest costs for a given output power requirement or specification. Resonant power converters are particularly useful for high switching frequencies such as frequencies above 1 MHz where switching losses of standard SMPS topologies (Buck, Boost etc.) tend to be unacceptable for conversion efficiency reasons. High switching frequencies are generally desirable because of the resulting decrease of the electrical and physical size of circuit components of the power converter like inductors and capacitors. The smaller components allow increase of the power density of the DC-DC power converter. In a resonant power converter an input “chopper” semiconductor switch (often MOSFET or IGBT) of the standard SMPS is replaced with a “resonant” semiconductor switch. The resonant semiconductor switch relies on resonances of a resonant network typically involving various circuit capacitances and inductances to shape the waveform of either the current or the voltage across the semiconductor switch such that, when state switching takes place, there is no current through or no voltage across the semiconductor switch. Hence power dissipation is largely eliminated in at least some of the intrinsic capacitances or inductances of the input semiconductor switch such that a dramatic increase of the switching frequency into the VHF range becomes feasible for example to values above 30 MHz. This concept is known in the art under designations like zero voltage and/or zero current switching (ZVS and/or ZCS) operation. Commonly used switched mode power converters operating under ZVS and/or ZCS are often described as class E, class F or class DE inverters or power converters.
However, it remains a significant challenge to adjust or control the output power/voltage/current of resonant DC-DC power converters in an efficient way. If the resonant power converter is controlled by Pulse Width Modulation (PWM) of the “resonant” semiconductor switch, the ZVS ability is lost and power conversion efficiency will drop significantly. Varying the switching frequency of the resonant power converter has also been applied in prior art power converters to control the output voltage/current of the resonant power converter, but this control methodology suffers from a limited range of output voltage regulation and increasing power conversion losses. Controlling the output voltage/current of the resonant power converter by a control scheme which is a combination of variable switching frequency and PWM has also been applied in existing resonant power converters and generally proved to work well. This control methodology or scheme unfortunately leads to highly complex control circuitry.
Another more simple yet efficient way of controlling or adjusting the output power/voltage/current of resonant DC-DC power converters has been to turn on and off the entire resonant power converter in an intermittent manner. This control scheme is designated “burst mode control” or “on/off control”. Burst mode control allows the resonant power converter to operate at a fixed switching frequency where the conversion efficiency is high or optimal during on or activate time periods. During time periods where the power converter is off or deactivated, power losses are essentially eliminated because of the lack of switching activity of the resonant transistor which drives the resonant power converter. Ideally burst mode control of resonant power converter leads to full load regulation and constant efficiency from zero to full load on the converter.
On/off control of prior art resonant power converters has been achieved by controlling the signal voltage on the control terminal of a “resonant” semiconductor switch, e.g. a MOSFET gate terminal. This scheme may work in a satisfactory manner in some applications, but in order to regulate or adjust the converter output voltage and current a feedback control signal from the output/secondary side of the converter to the control terminal of the resonant” semiconductor switch is required. This presents a significant problem in isolated resonant power converters because the feedback control signal must cross a galvanic isolation barrier between the primary side circuitry and the secondary side circuitry. Traditionally, to maintain the galvanic isolation between input side circuitry and output side circuitry of the resonant power converter, the control signal to the resonant semiconductor switch has been transmitted through a relatively slow and expensive optocoupler or through a bulky and slow transformer. The time delay through the optocoupler or transformer presents, however, a serious obstacle to on/off control of resonant power converters where a fast transient response is highly desirable to provide adequate control of the converter output voltage and current. The time delay problem is particularly pronounced for high frequency resonant power converters operating with switching frequencies at or above 20 MHz.
TSO-SHENG CHAN ET AL: “A Primary Side Control Method for Wireless Energy Transmission System”, IEEE Transactions on Circuits and Systems i: regular papers, IEEE, Vol. 59, No. 8 discloses a wireless energy transmission system (WETS) transferring power from a primary side circuit to a secondary side circuit through a skin barrier. The IEEE paper discloses a resonant class E based DC-DC power converter with an inductive power transformer connecting the input side circuit and output side circuit through the skin barrier. A charging protection circuit comprise a controllable secondary side switch (Ms) which selectively connects and disconnects a battery (Vb) load from the output of the power converter. A primary side controller operates by detecting variations of the input current and phase of the input reactance to determine the state of the secondary side switch (Ms). The proposed range of switching frequencies of the class E based DC-DC power converter is between 83-175 kHz.
In view of these problems and challenges associated with prior art resonant power converters, it would be advantageous to provide a novel control mechanism for on/off control of resonant power converter eliminating the need to transmit the feedback control signal from an output voltage control circuit across a galvanic isolation barrier to the control terminal of the resonant semiconductor switch. The elimination of the feedback control signal would also be advantageous in non-isolated resonant power converters because of the time delay and occupation of board or carrier area associated with wiring of the feedback control signal to the resonant transistor.
In view of the above, it remains a challenge to reduce the size and lower component costs of both isolated and non-isolated resonant DC-DC power converters. It also remains a challenge to provide an output voltage control mechanism with fast transient response to provide good regulation of the converter output voltage even for high frequency resonant power converters. Hence, a novel control mechanism for resonant power converters which simplifies the control of the converter output voltage and reduces the number of electronic components required to perform the output voltage regulation is highly desirable.